Cellular studies have indicated that both mitochondrial and cytosolic aconitase can be inactivated by nitric oxide. These processes are very important metabolically. The former is essential for energy production via the Krebs cycle, and the latter in relation to iron regulation in the cells. The kinetics of reactions have been studied by ESR and correlated to the results of enzyme assays. Studies to characterize and quantitate the ESR-detectable species produced have been carried out.